It has been known that polymerization of carbon monoxide with an olefin such as ethylene and propylene, in the presence of a complex of a transition element such as palladium and nickel as a catalyst, gives a polyketone in which carbon monoxide and an olefin are substantially and completely alternately copolymerized (Industrial Material, 5, Dec., (1997)).
Because polyketone fibers show a high strength and a high elastic modulus, and are excellent in dimensional stability and bondability, they are expected to be applied to industrial material fibers such as reinforcing fibers for tires and belts, and reinforcing fibers for plastics. For example, research in automobile tires has been intensively carried out in recent years, and it has been considered that use of organic fibers having a small specific gravity compared with that of a steel and showing a high strength and a high elastic modulus as a reinforcing material for tires is effective in energy saving.
Polyketone fibers are fibers showing a high tensile strength and a high tensile elastic modulus as well as organic fibers extremely excellent in bondability to a rubber. Polyketone fibers are a new material highly adaptable for use as a rubber-reinforcing material such as tire cords.
In particular, a polyketone formed out of a repeating unit of ethylene and carbon monoxide is similar to a polyethylene and a poly(vinyl alcohol) in that the molecular chain of a polyketone can take an in-plane zigzag conformation. As a result, the molecular chain can be highly oriented by drawing to give fibers showing a high tensile strength and a high tensile elastic modulus to such a degree that the fibers are termed super fibers. Moreover, the polyketone fibers thus obtained are excellent in retention of a tensile strength and a tensile elastic modulus and dimensional stability at high temperature.
However, because a polyketone is drastically modified in a molten state, melt spinning a polyketone is difficult. Dry spinning or wet spinning in which forming is conducted while a polyketone is being dissolved in a solvent has, therefore, been investigated as a method of forming the polyketone into fibers.
For example, Japanese Unexamined Patent Publication (Kokai) No. 2-112413 discloses that polyketone fibers showing a tensile strength of 13.0 cN/dtex and a tensile elastic modulus of 206 cN/dtex are obtained by carrying out wet spinning with an organic solvent such as hexafluoroisopropanol or m-cresol, and hot drawing the spun yarn at a high draw ratio.
Furthermore, the pamphlets of International Publication Nos. 99/18143 and 00/09611 disclose methods of spinning a polyketone with an aqueous metal salt solution. For example, polyketone fibers showing a tensile strength of 11.6 cN/dtex and a tensile elastic modulus of 275 cN/dtex are obtained by wet spinning with an aqueous zinc chloride/sodium chloride solution used as a solvent, and hot drawing the resultant yarn at a high draw ratio.
Methods of spinning a polyketone with an aqueous metal salt solution used as a solvent are considered to be preferable for the following reasons. The solvent is prepared at low cost, is non-combustible, shows low toxicity, and is excellent in safety in the production process, spinning stability and solvent recovery.
Furthermore, in order to increase a tensile strength and a tensile elastic modulus of polyketone fibers, various spinning methods have been investigated. For example, Japanese Patent Publication No. 4-505344 discloses a wet spinning method in which an organic solvent is used, and by which polyketone fibers showing a tensile strength of 12 cN/dtex or more and a tensile elastic modulus of 250 cN/dtex or more are obtained by hot drawing a yarn of polyketone fibers that has an organic solvent as a residue prior to drawing. Moreover, the pamphlet of International Publication No. 02/068738 discloses, as a wet spinning method in which an aqueous metal salt solution is used as a solvent, a method of producing polyketone fibers showing a tensile strength of 12 cN/dtex or more and a tensile elastic modulus of 250 cN/dtex or more, the method comprising gel spinning a polyketone solution (spinning stock solution) having a phase separation temperature, cleaning, drying and hot drawing the spun yarn.
However, it has been found that modification of a polyketone proceeds by the formation of a furan ring in a high temperature atmosphere through a Paal-Knorr reaction, or by a chemical reaction such as an intramolecular or an intermolecular crosslinking by aldol condensation. Even a slight amount of a modified polymer generated during the production process of polyketone fibers causes yarn breakage and fluff formation during hot drawing. When the polymer modification is significant, the tensile strength and the tensile elastic modulus of the yarn are sometimes lowered. In particular, it has been found that when a polyketone is in an oxidizing, acidic or basic atmosphere and, for example, when a polyketone is contacted with a solvent in the above atmosphere, a problem, that a polyketone is likely to be modified even at low temperatures, arises.
In wet spinning with an organic solvent, in order to obtain polyketone fibers showing a still higher tensile strength and a still higher tensile elastic modulus, polyketone fibers must be drawn at high temperatures exceeding 150° C. while hexafluoroisopropanol remains in the fibers prior to drawing even when a solvent such as hexafluoroisopropanol that dissolves a polyketone at relatively low temperatures is used (Japanese Patent Publication No. 4-505344). However, because such a method accelerates modification of a polyketone, a problem, that the fibers thus obtained often show fluff formation and yarn breakage, arises. Moreover, when a solvent of m-cresol or resorcin/water is used, the problem that polymer modification is produced arises because the polyketone dissolves in the solvent at high temperatures exceeding 80° C.
Even in wet spinning with an aqueous metal salt solution used as a solvent, there is a problem about polymer modification, and various examinations have been performed to inhibit the modification. For example, the pamphlet of International Publication No. 00/09611 discloses that, in wet spinning with an aqueous metal salt solution used as a solvent, reduction of amounts of palladium, etc., that are residues of the polymerization catalyst and are contained in the polyketone and reduction of amounts of zinc, etc, that are components of the solvent and are contained therein decrease polymer modification produced during hot drawing, improve a tensile strength and a tensile elastic modulus, and are effective in suppressing yarn breakage. It is considered that the results are obtained because palladium and zinc are substances that accelerate modification of a polyketone with heat. Moreover, Japanese Unexamined Patent Publication (Kokai) No. 2001-123326 discloses that when polyketone fibers are allowed to contain specific additives to inhibit polymer modification and lowering of a polymer molecular weight during hot drawing, the effect of improving a tensile strength is obtained.
However, in the process for producing polyketone fibers with an aqueous metal salt solution used as a solvent, a polymer modification produced by heating a polyketone with a solvent particularly causes disadvantages. When conventional technologies as explained above alone are applied, a variation in the tensile strength of the resultant polyketone fibers is sometimes increased, and the heat resistance thereof is sometimes deteriorated.
Furthermore, for example, in the step of twisting a yarn of polyketone fibers in order to use the yarn as tire cords that are a principal application of the fibers, the stability of the yarn during treatment of the yarn after the production of the fibers sometimes causes problems, for example, fluffs in the yarn are often formed. When fluffs are formed during twisting the yarn, problems, such as lowering of the cord tenacity and significant lowering of the tensile strength of the cords that are in rubber products such as tires and that suffer repeated elongation and compression, arise.
Moreover, in order to increase the productivity of polyketone fibers while the installation cost is being taken into consideration, drawing the yarn at a high speed with a heating zone that is made as short as possible is advantageous, that is, hot drawing at a high strain rate is advantageous. However, in the conventional production process, hot drawing the yarn at a high strain rate in a high draw ratio range where the physical properties of a high tensile strength and a high elastic modulus are manifested causes the following problems: fluffs are formed during drawing; the tensile strength and tensile elastic modulus are markedly lowered in comparison with the yarn of polyketone fibers that is drawn at a low strain rate in the same draw ratio.
Furthermore, the present inventors have found that in order to inhibit the polyketone modification in a solvent, it is extremely important to adjust the temperature and period of not only the step of dissolving the polyketone but also the steps in which the polyketone is contacted with a solvent until the solution is injected into a coagulation bath. However, although preceding references such as the pamphlets of International Publication Nos. 99/18143, 00/09611 and 02/068738 and Japanese Unexamined Patent Publication (Kokai) No. 2001-123326 describe the temperature and period in the dissolution step, they do not describe at all the conditions in steps subsequent to the dissolution such as a filtering step and a solution-feeding step in a tubing. The above fact indicates that no one has recognized that, in order to industrially stably produce polyketone fibers, inhibition of a polymer modification in a solvent is an extremely important factor.
Furthermore, it is effective to measure a UV absorbance as an index of a polymer modification in a trace amount produced by heating a polyketone contacted with a solvent. However, no preceding references exist that describe the relationship between a degree of a polymer modification shown by a UV absorbance and properties of the polyketone fibers.